Use of renewable energy resources is widely increasing globally. Renewable energy sources such as wind energy, wave energy, solar energy, fuel cells, and the like are characterized in that the energy supply is not constant, for example the wind and sunlight are almost constantly varying. This provides a challenge when integrating renewable energy sources into power supply grids, such as an electrical power grid (utility grid), wherein the requirements to real and reactive power, frequency, voltage and the like typically will be precisely defined. Therefore a plurality of methods has been developed to integrate and synchronize these variable energy sources into the electrical power grid. A widespread method is to use a converter to convert the variable electrical energy signal from the source to a constant AC current signal suited for the electrical power grid. The use of converters is common in relation to wind energy due to the type of generators used in wind turbines. Regulation and control of modern converters are typically based on or are variations of the regulation described by Pena, C et al. (1996) Doubly fed induction generator using back-to-back PWM converters and its applications to variable-speed wind-energy generation, IEE Proc.-Electr. Power Appl., 143, 231-241.
Due to the regulation in a converter an excess of reactive power is created and a commonly used method of regulating this reactive power is to apply a reactor (inductor) between the converter and the grid. The reactor is then said to consume the reactive power (at least a part of it) from the converter by acting as a filter. A reactor typically comprises an iron core with copper windings and the physical requirements scales with the current flowing trough the reactor. Thus, larger maximum current of the reactor requires larger iron core and larger (and/or added) copper windings. If the converter is used to connect an electrical energy source, such as a wind turbine, to the utility grid, the requirements to the size of the reactor scales with the power of the energy source, i.e. large power sources require large reactors to handle the large currents.